1. Field of the Invention
This invention is concerned with a method for suppressing extraneous noise from seismic recordings. In particular the method is concerned with suppressing signals due to shots generated by an independently operating seismic exploration crew.
2. Description of the Prior Art
In marine seismic exploration, a seismic ship tows a streamer cable including a plurality of seismic receivers such as hydrophones through a body of water. The streamer cable is generally towed at a desired depth beneath the water surface, such as 10 meters. Generally the streamer cables provide 96 or more data channels and may be three kilometers or more in length. As the ship proceeds along a line of survey, an acoustic source is triggered to produce an impulse at spaced apart intervals of 25 to 50 meters. The acoustic waves from the impulse travel downwardly into the earth beneath the water. The waves are reflected from various earth layers back to the water surface where they are sensed by the receivers, converted to electrical signals as a function of total travel time, and recorded for future processing. A similar course of events takes place during land surveying, except of course, land vehicles replace ships.
Some areas of the world, such as the Gulf of Mexico, are heavily explored so that several seismic ships, working for unrelated operators may be conducting surveys at the same time. The receivers used in seismic streamer cables are quite sensitive as well as being omnidirectional. Any sound that the receiver hydrophones hear is detected and recorded. Thus, the receivers respond not only to an impulse or "shot" generated by their own ship, but may also respond to shots generated or fired by another ship. For any given ship, shots from other ships are necessarily coherent noise interference but generated at time intervals unrelated to those of the impulses of the given ship.
It is known of course, that the energy level of the first, direct water-arrivals from a shot may be 80 to 120 dB higher than the much weaker reflected arrivals that arrive several seconds later. It is also well known that the water volume between the water surface and the sea floor acts as an acoustic wave guide. Because of that phenomenon, the first direct signal arrivals from a ship as far as 50 to 100 kilometers distant may create a serious noise problem for some other ship. The problem of other-ship noise contamination is so serious that the respective operators within a given area allot daily time periods amongst themselves so that each operator can conduct his studies in comparative quiet. Preferably a marine seismic crew operates 24 hours per day to maximize revenue time against fixed overhead. Any involuntary standby time is very costly.
Most classical seismic data processing methods involving noise suppression are based upon the assumption that ambient noise events on any given shot record are random and incoherent. The noise level is reduced by coherent "stacking" or summation after application of various well-known processes of common-midpoint processing and the like.
Other-ship noise is coherent. A single shot from another ship will contaminate all of the traces of an own-ship seismic record. Coherent noise is not amenable to conventional treatment. This disclosure addresses itself to the problem of removing other-ship coherent noise.